Inferential modality with mora da

You already use morati for what someone has to do and moći for what someone can do. Serbian uses the same two verbs for something quite different: to say how sure you are that something is true. When you see wet streets and say "it must have rained", nobody has to do anything — you are drawing a conclusion. This is called inference, and this lesson is about saying it out loud.

The shape: a frozen mora + da + a whole sentence

For a conclusion, mora does not change. It stays in this one form whoever you are talking about, and everything you conclude follows da as a complete sentence with its own subject and its own tense: Mora da je Jelena bolesna. (Jelena must be ill.) Mora da su zaboravili na sastanak. (They must have forgotten about the meeting.) Think of it as "It must be that ...".

Must have done: the most useful one

English puts the past onto must: "he must have left". Serbian keeps mora in the present — you are concluding now — and puts the past into the sentence after da: Mora da je otišao. The perfect after da agrees with its own subject as usual: mora da je zaboravio (he), mora da je zaboravila (she), mora da su zaboravili (they), mora da sam zaboravio (I, man speaking).

Do not translate "must have done" with morao je da. Morao je da ode means he HAD TO leave. For a guess about the past, keep mora as it is and put the past after da: Mora da je otišao. And never make that mora agree: Mora da su umorni (they must be tired), not moraju da su.

Mora biti da

Mora biti da means exactly the same as mora da, with the infinitive biti added: "it must be (the case) that". It sounds a little more deliberate, the way a detective sums up: Mora biti da je vozač zaspao. (The driver must have fallen asleep.) Everything after da works the same way.

Ne može: it can't be

To say something is impossible, use ne može with da bude (to be): On ne može da bude lopov. (He can't be the thief.) To ne može da bude istina. (That can't be true.) For a past event, use ne može biti da plus the perfect: Ne može biti da je Marko to uradio. (Marko can't have done that.)

Do not use ne mora for "can't". Ne mora da je on means "it isn't necessarily him" — a much weaker claim. When you are sure something is NOT true, it is ne može.

Moglo bi da: it might

The conditional of moći from the lesson on bih gives the cautious version. Used for a conclusion it is frozen in the neuter, moglo bi, followed by da and a whole sentence: Moglo bi da je ona u pravu. (She might be right.) Moglo bi da padne kiša. (It might rain.)

Asking whether it could be

In a question, moći usually agrees with the thing you are guessing about: Može li to da bude njegov auto? (Could that be his car?) Ko bi to mogao da bude? (Who could that be?) Gde bi mogle da budu moje naočare? (Where could my glasses be? — naočare is feminine plural, so mogle.)

A later lesson adds adverbs such as sigurno and verovatno, which grade certainty without a modal verb. Here the certainty sits in mora, može and moglo bi.